The default tabbing order for objects in a form is from left to right, top to bottom, starting from the upper-left corner. Tabbing order respects the existence of subforms, radio buttons, and content areas. For example, if two subforms exist side-by-side, and each subform contains a number of field objects, the tabbing sequence will go through the fields in the first subform before moving on to the next.
Note:
LiveCycle Designer ES does not include circle, line, or rectangle objects in the tabbing order.
The tabbing order starts from the object with the smallest vertical coordinate and ends with the object with the largest vertical coordinate, regardless of whether the object is on the body or master page.
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Content area object on the page with a vertical coordinate of 4. All objects in the content area are tabbed through before tabbing to the subform object on the master page.
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Subform object on the master page with a vertical coordinate of 10. All objects in the content area are tabbed through before tabbing to the text object on the master page.
Because the tabbing order is important, ensure that you position objects precisely on the form, relative to each other. For example, you can position and size an object using its coordinates and you can snap objects to points on a grid.
You can change the default tabbing order if you require a different sequence in your form. For example, you may want to change the tabbing order to move through objects in a column, from top to bottom, and then left to right.
Note:
In Acrobat 6.0.2, tabbing to a group of radio buttons makes the upper-left radio button active. Use the Tab key to move through the radio buttons. In Acrobat 7.0.5 and later, the selected radio button becomes active. Use the arrow keys to move between the radio buttons in a group and the Tab key to move out of the group.
See also 

How the default tabbing order works